Reactions of Trialkyl Phosphates with Trialkyls of Aluminum and

Jan 20, 1999 - Trialkyl phosphates OP(OR)3 (R = Me, Et, Ph, SiMe3) react with AlMe3, AlEt3, and GaMe3 in hydrocarbon solvents to form the adducts Me3A...
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Organometallics 1999, 18, 523-528

523

Reactions of Trialkyl Phosphates with Trialkyls of Aluminum and Gallium: New Route to Alumino- and Gallophosphate Compounds via Dealkylsilylation† Jiri Pinkas,‡ Debashis Chakraborty, Yu Yang, Ramaswamy Murugavel, Mathias Noltemeyer, and Herbert W. Roesky* Institut fu¨ r Anorganische Chemie der Georg-August-Universita¨ t, Go¨ ttingen, Tammannstrasse 4, D-37077 Go¨ ttingen, Germany Received August 4, 1998

Trialkyl phosphates OP(OR)3 (R ) Me, Et, Ph, SiMe3) react with AlMe3, AlEt3, and GaMe3 in hydrocarbon solvents to form the adducts Me3Al‚OP(OR)3 (R ) Me (1), Et (2), Ph (3), SiMe3 (4)), Et3Al‚OP(OR)3 (R ) SiMe3 (5)), and Me3Ga‚OP(OR)3 (R ) Me (6), SiMe3 (7)). These products were characterized by 1H, 13C, 29Si, and 31P NMR, infrared (IR), and mass (MS) spectroscopies and elemental analysis. The trimethylsilyl ester adducts 4, 5, and 7 further undergo a thermally induced dealkylsilylation with the formation of cyclic aluminoand gallophosphate dimers, [R′2M(µ2-O)2P(OSiMe3)2]2 (R′2M ) Me2Al (8), Et2Al (9), and Me2Ga (10)). The molecular structures of 8 and 9 were confirmed by X-ray crystallography.

Introduction We are interested in developing rational synthetic techniques leading to microporous materials, such as alumino- and gallophosphates, using nonaqueous methods. In the pursuit of this goal, we followed the building block strategy and synthesized a series of molecular alumino- and gallophosphonates. These compounds represent organic-soluble models of the secondary building units (SBU,1 Scheme 12) found in extended threedimensional structures of AlPO and GaPO materials.3 To the previously known model compounds, mimicking a single four-ring4-10 (4R) and a double four-ring 10-13 (D4R), we added a double six-ring14 (D6R), a single sixring15 (6R), and a tricyclic unit (6t1).16 * To whom correspondence should be addressed. † Dedicated to Professor Heinrich Marsmann on the occasion of his 60th birthday. ‡ Current address: Department of Inorganic Chemistry, Masaryk University, Kotlarska 2, 61137 Brno, Czech Republic. (1) (a) Meier, W. M.; Olson, D. H. Atlas of Zeolite Structure Types, 3rd revised ed.; Butterworth-Heinemann: London, 1992. (b) Smith, J. V. Chem. Rev. 1988, 88, 149. (c) van Konigsveld, H. In Introduction to Zeolite Science and Practice; van Bekkum, H., Flanigen, E. M., Jansen, J. C., Eds.; Stud. Surf. Sci. Catal. 1991, 58, 35. (2) Each SBU edge in Scheme 1 represents a M-O-P (M ) Al, Ga) linkage. Organic substituents on the vertexes are omitted. (3) (a) Oliver, S.; Kuperman, A.; Ozin, G. A. Angew. Chem. 1998, 110, 48; Angew. Chem., Int. Ed. Engl. 1998, 37, 46. (b) Estermann, M.; McCusker, L. B.; Baerlocher, C.; Merrouche, A.; Kessler, H. Nature 1991, 352, 320. (4) The terms four-ring and six-ring refer to the number of tetrehedral atoms (Al, Ga, P) in the ring. (5) (a) Coates, G. E.; Mukherjee, R. N. J. Chem. Soc. 1964, 1295. (b) Weidlein, J.; Schaible, B. Z. Anorg. Allg. Chem. 1971, 386, 176. (c) Schaible, B.; Weidlein, J. J. Organomet. Chem. 1972, 35, C7. (d) Olapinski, H.; Schaible, B.; Weidlein, J. J. Organomet. Chem. 1972, 43, 107. (6) Sangokoya, S. A.; Pennington, W. T.; Robinson, G. H.; Hrncir, D. C. J. Organomet. Chem. 1990, 385, 23. (7) Hahn, F. E.; Schneider, B.; Reier, F.-W. Z. Naturforsch. 1990, 45B, 134. (8) (a) Landry, C. C.; Hynes, A.; Barron, A. R.; Haiduc, I.; Silvestru, C. Polyhedron 1996, 15, 391. (b) Keys, A.; Bott, S.; Barron, A. R. Chem. Commun. 1996, 2339.

Scheme 1

We also employed a complementary strategy for the preparation of these materials in which aluminum and gallium amides were reacted with trialkyl phosphates. These reactions of trifunctional moieties provide amorphous alumino- and gallophosphates under totally nonaqueous conditions via a dealkylamination route.17 We surmised that species with structures analogous to 4R and D4R presumably are intermediates in these reactions (eq 1). (9) (a) Browning, D. J.; Corker, J. M.; Webster, M. Acta Crystallogr. 1996, C52, 882. (b) Corker, J. M.; Browning, D. J.; Webster, M. Acta Crystallogr. 1996, C52, 583. (10) (a) Mason, M. R.; Matthews, R. M.; Mashuta, M. S.; Richardson, J. F. Inorg. Chem. 1996, 35, 5756. (b) Mason, M. R.; Mashuta, M. S.; Richardson, J. F. Angew. Chem. 1997, 109, 249; Angew. Chem., Int. Ed. Engl. 1997, 36, 239. (c) Mason, M. R. J. Cluster Sci. 1998, 9, 1. (11) Yang, Y.; Schmidt, H.-G.; Noltemeyer, M.; Pinkas, J.; Roesky, H. W. J. Chem. Soc., Dalton Trans. 1996, 3609. (12) Cassidy, J. E.; Jarvis, J. A. J.; Rothon, R. N. J. Chem. Soc., Dalton Trans. 1975, 1497. (13) Walawalkar, M. G.; Murugavel, R.; Roesky, H. W.; Schmidt, H.-G. Inorg. Chem. 1997, 36, 4202. (14) Yang, Y.; Walawalkar, M. G.; Pinkas, J.; Roesky, H. W.; Schmidt, H.-G. Angew. Chem. 1998, 110, 101; Angew. Chem., Int. Ed. Engl. 1998, 37, 96. (15) Yang, Y.; Pinkas, J.; Noltemeyer, M.; Roesky, H. W. Inorg. Chem. 1998, 37, 6404. (16) Yang, Y.; Pinkas, J.; Schaefer, M.; Roesky, H. W. Angew. Chem. 1998, 110, 2795; Angew. Chem., Int. Ed. Engl. 1998, 37, 2650. (17) Pinkas, J.; Wessel, H.; Yang, Y.; Montero, M. L.; Noltemeyer, M.; Froeba, M.; Roesky, H. W. Inorg. Chem. 1998, 37, 2450.

10.1021/om9806702 CCC: $18.00 © 1999 American Chemical Society Publication on Web 01/20/1999

524 Organometallics, Vol. 18, No. 4, 1999

1/2[M(NMe2)3]2 + OP(OR)3 f (Me2N)3M‚OP(OR)3 (1) f [(Me2N)2M(µ2-O)2P(OR)2]n + RNMe2 f [(Me2N)M(µ2-O)3P(OR)]n + RNMe2 f MPO4 + RNMe2 M ) Al, Ga; R ) Me, Et, n-Bu, SiMe3 To gain further insight into the mechanism of these reactions and to be able to analyze their individual steps, we prepared and fully characterized a series of adducts Me3Al‚OP(OR)3 (R ) Me (1), Et (2), Ph (3), SiMe3 (4)), Et3Al‚OP(OR)3 (R ) SiMe3 (5)), and Me3Ga‚ OP(OR)3 (R ) Me (6), SiMe3 (7)). These compounds will serve as models for identification of nonisolable initial intermediates (Me2N)3M‚OP(OR)3 in the reaction mixture (eq 1) in the low-temperature NMR spectroscopic studies. Interestingly, the tris(trimethylsilyl) phosphate adducts 4, 5, and 7 undergo a thermally induced dealkylsilylation and form cyclic phosphate dimers [R′2M(µ2O)2P(OSiMe3)2]2 (R′2M ) Me2Al (8), Et2Al (9), and Me2Ga (10)). Dealkylsilylation is a rare reaction which has been employed previously in the preparation of 12/ 1618 and 13/1519 materials. Compounds 8 and 9 were shown to possess the cyclic dimeric structure by NMR spectroscopy and X-ray crystallography and thus constitute additional examples of organic-soluble molecular models of the ubiquitous 4R SBU. Furthermore, these findings lend support to our mechanistic contention regarding the amide-phosphate systems17 that four-ring units are one of the intermediates along the reaction pathway toward MPO4 (M ) Al, Ga). Experimental Section All preparative procedures were performed under a dry nitrogen atmosphere using Schlenk and drybox techniques. Solvents were dried over and distilled from Na/benzophenone under nitrogen. Deuterated solvents were dried over and distilled from Na/K alloy and degassed prior to use. 1H, 13C, 29Si, and 31P NMR spectra were measured on Bruker MSL400, AM-250, and AM-200 instruments. Mass spectra were obtained on a Finnigan MAT 8230 or MAT 95 mass spectrometer (EI, 70 eV or FI). IR spectra (4000-400 cm-1) were recorded on Bio Rad FTS-7. Samples of solids were prepared as KBr pellets, while neat liquids were spread between KBr disks. Elemental analyses were carried out by the Analytisches Labor des Anorganischen Instituts, Go¨ttingen. Poor analysis results for compounds 1 and 3 are due in part to metal carbide formation and also to the pyrophoric nature of the adducts. Melting points were measured in sealed capillaries and are uncorrected. General Procedure for the Reactions of Trialkyl Phosphates with Trimethylaluminum and Trimethylgallium. The phosphate ester (10 mmol) was dissolved in dry hexane (50 mL) or, in the case of OP(OPh)3, in toluene, and AlMe3 (2.0 M in hexane, 10% excess over the equimolar amount) was added slowly via syringe to the stirred and ice(18) Stuczynski, S. M.; Brennan, J. G.; Steigerwald, M. L. Inorg. Chem. 1989, 28, 4431. (19) (a) Stuczynski, S. M.; Opila, R. L.; Marsh, P.; Brennan, J. G.; Steigerwald, M. L. Chem. Mater. 1991, 3, 379. (b) Dillingham, M. D. B.; Burns, J. A.; Byers-Hill, J.; Gripper, K. D.; Pennigton, W. T.; Robinson, G. H. Inorg. Chim. Acta 1994, 216, 267. (c) Barry, S. T.; Belhumeur, S.; Richeson, D. S. Organometallics 1997, 16, 3588.

Pinkas et al. cooled solution. The reaction mixture was left to warm slowly to ambient temperature. After solvent volume reduction under vacuum, a white solid (1 and 4) formed on storing at -24 °C. The products were cold filtered and dried under vacuum for 2 h. Compounds 2, 3, and 5, on the other hand, were obtained as pure colorless liquids upon removing all volatile components under vacuum. A colorless liquid 6 and a white solid 7 were obtained by adding a phosphate ester to the solution of GaMe3 (6 mmol, 7% excess over the equimolar amount) in hexane at room temperature. Me3Al‚OP(OMe)3 (1). Yield: 74%. Mp: 34.5-35.0 °C. 1H NMR (benzene-d6): δ -0.398 (s, AlCH3, 9 H), 3.166 (d, 3 JPH ) 11.5 Hz, POCH3, 9 H). 13C NMR (benzene-d6): δ -6.76 (br s, AlCH3), 55.53 (d, 2JPC ) 6.2 Hz, POCH3). 31P NMR (benzene-d6): δ -3.19 (s). MS (FI): m/z (%) 197 (M - CH3, 100), 155 (30), 140 (OP(OMe)3, 55). Anal. Calcd for C6H18AlO4P: C, 33.97; H, 8.55; Al, 12.72. Found: C, 32.84; H, 7.95; Al, 12.7. Me3Al‚OP(OEt)3 (2). Yield: 86%. 1H NMR (benzene-d6): δ -0.597 (s, AlCH3, 9 H), 0.964 (dt, 4JPH ) 1.3 Hz, 3JHH ) 7.1 Hz, POCH2CH3, 9 H), 3.786 (dq, 3JPH ) 8.3 Hz, 3JHH ) 7.1 Hz, POCH2CH3, 6 H). 13C NMR (benzene-d6): δ -6.98 (br s, AlCH3), 15.57 (d, 3JPOCC ) 6.7 Hz, POCH2CH3), 66.25 (d, 2JPC ) 6.4 Hz, POCH2CH3). 31P NMR (benzene-d6): δ -6.91 (s). MS (EI): m/z (%) 239 (M - CH3, 17), 155 (P(OEt)2(OH)2, 100). MS (FI): m/z (%) 239 (M - CH3, 100). IR (neat, KBr disks, cm-1): ν 2988 s, 2918 s, 2884 s, 2817 m, 1480 m, 1445 m, 1398 m, 1373 m, 1294 m, 1224 vs (ν PdO), 1177 s, 1103 w, 1044 vs, 994 w, 825 s, 808 s, 697 vs, 615s, 518 m, 487 w. Anal. Calcd for C9H24AlO4P: C, 42.52; H, 9.51; Al, 10.61. Found: C, 42.26; H, 8.89; Al, 10.3. Me3Al‚OP(OPh)3 (3). Yield: 92%. 1H NMR (benzene-d6): δ -0.420 (s, AlCH3, 9 H), 6.8-7.1 (m, Ph, 15 H). 13C NMR (benzene-d6): δ -6.77 (br s, AlCH3), 120.04 (d, 3JPC ) 4.9 Hz, o-C), 126.89 (d, 5JPC ) 1.4 Hz, p-C), 130.42 (d, 4JPC ) 1.0 Hz, m-C), 149.81 (d, 2JPC ) 8.3 Hz, ipso-C). 31P NMR (benzened6): δ -21.89 (s). MS (EI): m/z (%) 383 (M - CH3, 16), 326 (OP(OPh)3, 78), 281 (30), 249 (O2P(OPh)2, 45), 177 (26), 161 (100). IR (neat, KBr disks, cm-1): ν 3062 w, 3045 w, 2919 s, 2884 m, 2816 w, 1589 s, 1489 vs, 1458 m, 1266 s (ν PdO), 1219 m, 1180 vs, 1160 vs, 1072 m, 1029 vs, 992 vs, 953 w, 905 m, 779 m, 753 s, 704 vs, 685 vs, 617 m, 580 w, 519 m. Anal. Calcd for C21H24AlO4P: C, 63.32; H, 6.07; Al, 6.77. Found: C, 62.23; H, 6.02; Al, 6.5. Me3Al‚OP(OSiMe3)3 (4). Yield: 74%. Mp: 86-88 °C. 1H NMR (benzene-d6): δ -0.327 (s, AlCH3, 9 H), 0.148 (s, 1JCH ) 120.0 Hz, 13C satellites, 2JSiH ) 7.0 Hz, 29Si satellites, SiCH3, 27 H). 13C NMR (benzene-d6): δ -6.2 (br s, AlCH3), 0.31 (d, 3J 1 29 29 PC ) 2.0 Hz, JSiC ) 60.3 Hz, Si satellites, SiCH3). Si NMR (benzene-d6): δ 26.7 (d, 2JPSi ) 5.8 Hz, 1JSiC ) 60.4 Hz, 13C satellites). 31P NMR (benzene-d6): δ -31.7 (s, 2JPSi ) 5.7 Hz, 29Si satellites). MS (EI): m/z (%) 655 (5), 581 (8 - CH , 100). 3 MS (FI): m/z (%) 581 (8 - CH3, 10), 371 (M - CH3, 100). IR (KBr pellet, cm-1): ν 2963 m, 2927 w, 1422 w, 1260 s, 1221 s (ν PdO), 1130 m, 1082 vs, 853 vs, 764 m, 687 s, 615 m, 557 w, 467 m. Anal. Calcd for C12H36AlO4PSi3: C, 37.28; H, 9.38; Al, 6.98; P, 8.01. Found: C, 36.70; H, 9.20; Al, 7.02; P, 8.17. Et3Al‚OP(OSiMe3)3 (5). Yield: 87%. 1H NMR (benzened6): δ 0.135 (d, 4JPH ) 0.4 Hz, 1JCH ) 120.0 Hz, 13C satellites, 2J 29Si satellites, SiCH , 27 H), 0.295 (q, 3J SiH ) 6.9 Hz, 3 HH ) 8.1 Hz, CH2, 6 H), 1.555 (t, 3JHH ) 8.1 Hz, CH3, 9 H). 13C NMR (benzene-d6): δ 0.23 (d, 3JPC ) 1.8 Hz, 1JSiC ) 60.4 Hz, 29Si satellites, SiCH3), 1.78 (br s, AlCH2), 10.86 (s, CH3). 29Si NMR (benzene-d6): δ 26.6 (d, 2JPSi ) 6.1 Hz, 1JSiC ) 60.4 Hz, 13C satellites). 31P NMR (benzene-d6): δ -31.7 (s, 2JPSi ) 6.1 Hz, 29Si satellites). MS (EI): m/z (%) 623 (9 - C H , 20), 399 (M 2 5 C2H5, 68), 299 (OP(OSiMe3)3 - CH3, 100). MS (FI): m/z (%) 623 (9 - C2H5, 14), 399 (M - C2H5, 100). IR (neat, KBr disks, cm-1): ν 2963 s, 2932 s, 2897 vs, 2888 vs, 2853 vs, 2789 m, 1463 w, 1410 m, 1260 vs, 1215 vs (ν PdO), 1080 vs, 985 s, 943 m, 852 vs, 764s, 700 w, 646 s, 616 s, 475 w.

Reactions of Trialkyl Phosphates Me3Ga‚OP(OMe)3 (6). Yield: 75%. 1H NMR (benzene-d6): δ 0.007 (s, GaCH3, 1JCH ) 118 Hz, 13C satellites, 9 H), 3.185 (d, 3JPH ) 11.3 Hz, 1JCH ) 149 Hz, 13C satellites, POCH3, 9 H). 13C NMR (benzene-d6): δ -2.88 (s, GaCH3), 54.67 (d, 2JPC ) 6.1 Hz, POCH3).31P NMR (benzene-d6): δ -0.05 (s). MS (FI): m/z (%) 239 (M - CH3, 100). Me3Ga‚OP(OSiMe3)3 (7). Yield: 95%. Mp: 37-38 °C. 1H NMR (benzene-d6): δ 0.122 (s, GaCH3, 9 H), 0.146 (d, 4JPH ) 0.6 Hz, 2JSiH ) 6.8 Hz, 29Si satellites, SiCH3, 27 H). 13C NMR (benzene-d6): δ -2.70 (br s, GaCH3), 0.40 (d, 3JPC ) 1.8 Hz, 1 JSiC ) 60.3 Hz, 29Si satellites, SiCH3). 29Si NMR (benzened6): δ +24.23 (d, 2JPSi ) 5.4 Hz, 1JSiC ) 60.3 Hz, 13C satellites). 31 P NMR (benzene-d6): δ -28.5 (s, 2JPSi ) 5.3 Hz, 29Si satellites). MS (EI): m/z (%) 667 (10 - CH3, 100). MS (FI): m/z (%) 667 (10 - CH3, 100). IR (neat, KBr disks, cm-1): ν 2964 vs, 2906 m, 2840 w, 1420 w, 1259 s, 1213 vs (ν PdO), 1197 s, 1099 vs, 1065 vs, 849 vs, 763 s, 736 m, 612 m, 596 sh, 546 m, 481 w. Anal. Calcd for C12H36GaO4PSi3: C, 33.57; H, 8.45; Ga, 16.24; P, 7.21. Found: C, 33.36; H, 8.10; Ga, 16.70; P, 7.27. Preparation of the Dimeric Four-Ring Compounds 8-10. [Me2Al(µ2-O)2P(OSiMe3)2]2 (8). Method A. Compound 4 (12.41 g, 32.10 mmol) was heated in a sublimation apparatus under dry nitrogen at atmospheric pressure to 200 °C for 4 h. It underwent a dealkylsilylation and gave a white crystalline solid which condensed on the coldfinger. This product was subsequently resublimed at 120 °C/0.007 mm, affording 8.57 g (90%) of 8. The outlet of the sublimation apparatus was connected to a cold trap held at -70 °C. A colorless liquid which collected in the cold trap (1.70 g) was shown by 1H, 13C, and 29Si NMR spectroscopy to consist of mainly SiMe4 with traces of MeOSiMe3 ( 2σ(I)]a wR2 (all data)b GOF on F2 a

8

9

C16H48Al2O8P2Si4 596.80 19.309(4) 9.757(2) 19.846(4) 109.41(3) 3526.6(12) 4 C2/c (No. 15) -123(2) 0.710 73 1.124 0.90 × 0.70 × 0.30 3.39 4711 2306 0.0215 0.0360 0.0961 1.032

C20H56Al2O8P2Si4 652.91 18.581(5) 11.016(4) 21.442(6) 115.22(2) 3971(2) 4 P21/n (No. 14) -93(2) 0.710 73 1.092 0.70 × 0.60 × 0.60 3.07 12707 6877 0.0553 0.0850 0.2701 1.024

R1 ) Σ||Fo| - |Fc||/Σ|Fo|. b wR2 ) [Σw(|Fo2| - |Fc2|)2/Σw|Fo2|2]1/2.

s, 2792 w, 1458 w, 1411 w, 1370 vw, 1259 s, 1221 s (ν PdO), 1128 m, 1067 vs, 989 w, 948 w, 852 vs, 763 m, 696 w, 648 s, 613 w, 531 w, 467 w. Anal. Calcd for C20H56Al2O8P2Si4: C, 36.79; H, 8.65. Found: C, 36.27; H, 8.60. [Me2Ga(µ2-O)2P(OSiMe3)2]2 (10). A benzene-d6 solution of 7 was heated to 110 °C in a screw-cap NMR tube for 136 h. Approximately 50% of 7 was converted to 10. Byproducts included (Me3Si)2O and MeOSiMe3. 1H NMR (benzene-d6): δ 0.09 (s, GaCH3, 6 H), 0.229 (s, 1JCH ) 119 Hz, 13C satellites, 2J 29 13 SiH ) 6.9 Hz, Si satellites, SiCH3, 18 H). C NMR (benzene3 d6): δ -5.40 (t, JPC ) 2.6 Hz, GaCH3), 0.64 (d, 3JPC ) 1.8 Hz, 1J 29Si satellites). 29Si NMR (benzene-d ): δ 20.45 SiC ) 60 Hz, 6 (d, 2JPSi ) 5.0 Hz, 1JSiC ) 60 Hz, 13C satellites). 31P NMR (benzene-d6): δ -21.5 (s). Reaction of 2 with Dimethylamine. A solution of 2 (0.230 g, 0.905 mmol) in C6D6 (0.6 mL) was placed in a 200 mL ampule fitted with a Teflon valve. An excess of Me2NH (10 mL, dried over Na) was condensed into the ampule, and the reaction mixture was degassed by three freeze-pump-thaw cycles. Overnight heating to 80 °C yielded a white solid (88 mg). The excess of Me2NH was vented at room temperature, and all the remaining volatile components were vacuum transferred to an NMR tube and flame-sealed. EtNMe2 was identified by 1H and 13C NMR spectroscopy. Single-Crystal X-ray Diffraction Study of 8 and 9. Intensity data were collected on a Siemens-Stoe AED2 fourcircle diffractometer. The cell parameters were determined from randomly chosen and well-centered high-angle reflections. The structure solution by direct methods and refinement by full-matrix least-squares on F2 were carried out using SHELXS-90 and SHELXL-93 programs. All non-hydrogen atoms were refined anisotropically; the hydrogen atoms were found and refined isotropically. Crystal data and solution and refinement parameters are listed in Table 1.

Results and Discussion Adducts. Compounds 1-7 were obtained in high yield by direct reactions of their components in hydrocarbon solvents (eq 2) as liquids or low-melting solids. Their ester/metal alkyl composition (1:1) was confirmed by elemental analyses and a correct intensity ratio of the 1H NMR signals.

526 Organometallics, Vol. 18, No. 4, 1999

R′3M + OP(OR)3 f R′3M‚OP(OR)3

Pinkas et al.

(2)

M ) Al; R′ ) Me; R ) Me (1), Et (2), Ph (3), SiMe3 (4); M ) Al; R′ ) Et; R ) SiMe3 (5); M ) Ga; R′ ) Me; R ) Me (6), SiMe3 (7) The NMR spectra show that these adducts are stable in solution at room temperature. The 31P NMR chemical shifts of the AlMe3 adducts are shielded with respect to the free phosphate esters by 5-7 ppm, while the GaMe3 adducts only by 3-4 ppm. The smaller upfield shift of the latter derivatives reflects the lower acidity of the gallium center. The magnitudes of ∆δ values in the AlR′3 series correspond well with the published shielding (6.7 ppm) of the Cl3Al‚OP(OPh)3 signal upon adduct formation.20 The nature of the alkyl group on Al (Me or Et) induced no detectable change of the 31P NMR chemical shifts of 4 and 5. The coordination of a metal moiety to the phosphoryl group decreases its stretching frequency by 33-54 cm-1. These values agree well with the shifts reported for a series of AlMe3 adducts with phosphine oxides.21 However, we found little correlation between changes in the 31P NMR chemical shifts and the PdO vibrations. Some adducts were of limited stability under electron impact (EI-MS) conditions. The trimethyl phosphate adducts 1 and 6 showed no molecular peaks, while ethyl and phenyl ester adducts 2 and 3 showed them in low intensity (